Chelsea Clinton, the daughter of former president Bill Clinton and 2016 presidential candidate Hillary Clinton, wrote on Twitter that Mr Duterte was "a murderous thug with no regard for human rights" and that "rape is never a joke".

Image copyright@ChelseaClinton

Image copyright@ChelseaClinton

Phelim Kine of Human Rights Watch said the president's comment was "a sickening attempt at humour" that sent soldiers a signal they could commit rights abuses while enforcing martial law.

He said: "Duterte's pro-rape comments only confirm some of the worst fears of human rights activists that the Duterte government will not just turn a blind eye to possible military abuses in Mindanao, but may actively encourage them."

And a women's political party in the country, Gabriela, said in a statement: "Rape is not a joke. Martial law and the heightened vulnerability to military abuse that it brings to women and children are not a joke either."

Martial law was imposed last week on Mindanao in the south of the Philippines, where Muslim separatists and other rebels are fighting the army.

Last year, Mr Duterte joked about a 1989 rape and murder of an Australian missionary. He said that as mayor of the town where it happened, he should have been "first in line".

His spokesman Ernesto Abella said on Saturday that Mr Duterte was using "heightened bravado", giving "his full support to the men and women in uniform", and "taking complete responsibility for their actions".